1. Field of the Invention
The present invention relates to a circuit for detecting current flowing through an inductor.
2. Description of the Related Art
Inductors are used for various purposes as one fundamental element composing an electric circuit. For example, in a general DC/DC converter (switching regulator), an inductor is provided between a switching element and an output terminal, and is used to store energy.
FIG. 1 shows a conventional step-down type current mode DC/DC converter. This DC/DC converter generates output voltage Vout from input voltage Vin and regulates the output Vout.
Input voltage Vin is applied to switch M1, and switch M1 is turned on or off according to a control signal provided from a control circuit 1. A cathode of Diode D1 is connected to switch M1. Diode D1 operates as a rectification element. Inductor L is provided between switch M1 and the output terminal. Sense resistor Rs is connected to inductor L in series and is used as a shunt resistor. Output capacitor Cout smoothes the current to be supplied to a load. A voltage dividing circuit 2 divides output Vout at a prescribed rate.
The control circuit 1 generates a control signal for controlling switch M1 so as to maintain output Vout at a specific level, based on a voltage feedback signal and a current feedback signal. Here, the voltage feedback signal indicates output Vout and is obtained from the voltage dividing circuit 2. The current feedback signal indicates current flowing through inductor L (hereinafter called xe2x80x9cinductor currentxe2x80x9d) and the voltage across sense resistor Rs is used for the signal.
In this way, some DC/DC converters with an inductor detect output voltage or inductor current, and control a switch based on the detected value. In a conventional converter, as shown in FIG. 1, a resistor for detecting current (sense resistor Rs) was provided in series with the inductor, and the inductor current was detected based on the voltage across the resistor.
However, since a sense resistor is provided in series with the inductor, a voltage drop occurs. Thus, electric power is wasted. That is to say, if a sensor resistor is provided, the efficiency of the DC/DC converter deteriorates. In particular, since the output voltage of the DC/DC converter decreases as the voltage required by a load (for example, a CPU of a personal computer) decreases, a ratio of the voltage drop in the sense resistor to the output voltage of the DC/DC converter relatively increases. Therefore, loss in the sense resistor becomes a major cause preventing the efficiency of the DC/DC converter from being improved.
In addition, the sense resistor must have a small resistance value, have a wide temperature range under which it can be used, and have an accurate resistance value. As a result, cost of the sense resistor will be comparatively high.
This problem occurs not only in the DC/DC converter, but it also occurs when detecting current flowing through an inductor.
The objective of the present invention is to provide a circuit for detecting, with small loss, current flowing through an inductor.
The current detection circuit of the present invention detects current flowing through an inductor, and comprises a first circuit and a second circuit. The first circuit includes a resistor and a capacitor which are connected with each other in series, and is provided in parallel with the inductor. The second circuit detects current flowing through the inductor based on the voltage across the capacitor. A value obtained by multiplying the capacitance of a capacitor by the resistance value of the resistor is determined based on a ratio of the inductance of the inductor to the parasitic resistance value of the inductor. For example, if the inductance of the inductor, the parasitic resistance value of the inductor, the capacitance of the capacitor and the resistance value of the resistor are L, RL, Ca and Ra, respectively, the first circuit is designed in such a way that L/RL=Caxc2x7Ra is satisfied. Alternatively, the circuit constant of the first circuit is determined in such a way that the transfer function of voltage against current flowing through the inductor equals the transfer function of a capacitor voltage against current flowing in the first circuit.
In the above described circuit, the voltage across the capacitor equals the voltage across the parasitic resistance of the inductor. The voltage across the parasitic resistance of the inductor is a voltage drop generated by current flowing through the inductor. Therefore, if the parasitic resistance value of the inductor is known, current flowing through the inductor can be detected by monitoring the voltage across the capacitor. In particular, since a differential equation relating to the voltage across the parasitic resistance of the inductor equals the differential equation relating to the voltage across the capacitor under the condition that L/RL=Caxc2x7Ra is satisfied, the voltage across the capacitor always indicates the inductor current.